Negative inductance



Patented Apr. 11, 1933 UNITED STATES.

MIIIRAN M. DDLMAGE, OF WASHINGTON, DISTRICT OF COLUMBIA NEGATIVE INID'UG'IANCE Application filed October 4, 1928. Serial No. 310,395.

This invention relates to a new combina tion in the electrical art, a negative inductance.

.Where, in an electrical circuit or a transmission line, a single frequency alternating current is involved, it is Well known that the opposition offered to the flow of electrical currents by an inductance in the electrical circuit or in the transmission line may be offset by a capacity. The relief thus obtained may be partial or complete depending on the relative dimensions of the inductance, and the capacity. To .make this point perfectly clear,

1 5 consider a circuit containing in seriesan inductance.(L) and a capacity (C),ares1stance (R) and a generator having an electromotive force E sin pt. In such a circuit the reactance of the inductance is pL. The reactance of the capacity is i. p0 These-two rcactances being in phase opposition, the total net reactance of the circuit is given by Clearly, then, there is a frequency for which the two reactances, the capacity reactance and the inductive reactance, are equal to each other, and the net reactance is equal to zero.

Where, however, a number of frequencies have to be transmitted simultaneously, which is' the general case in signalling circuits, both wire and wireless, the difficulty arises that if the signalling circuit is tuned to some one 40 frequency, all other frequencies are adversely affected;

This invention relates to the discovery of a method for obtaining what can be properly called a negative inductance Full disclosure is made herein of the means required to carry out the method. Essentially, according to our definition of it, a negative inductance is 'a device which,in its effect, when interposed in an electrical circuit, is the exact counterpart of an ordinary inductance for Reactance pL Assuming such a negative inductance (;0L) to have been wired in series with an ordinary inductance (-l-pL) in an electrical G5 circuit, wherein acts an electromotive force of frequency (79), it follows immediately that the net reactance in thiscircuit is equal to (4:) pLpL=0. i

Hence in such a circuit, if a number of electromotive forces having dilferent frequencies (instead of a single frequency electromotive force) were active, the reactance of the circuit would be uniformly zero for P all frequencies as shown by Equation (4). There would be no discrimination as between different frequencies. To put this in another way, we would have realized a distortionless circuit. This is obviously true, since the above relation holds regardless of The practical importance, in signalling work of a negative inductance, having the characteristics hereinabove outlined, is too evident to require further comment.

It should be understood, of course, that instead of complete neutralization of the positive inductance of the circuit, we may arrange for partial neutralization, if we so Wish. The relative neutralization in such a case will be the same for all frequencies. If, for instance, we wish 50% neutralization of the positive inductance (37L) in the circuit, then the combination of the positive inductance and of the negative inductance will be represented by It'is necessary and sufficient in order to realize the above purpose that the value of the negative inductance be equal to The reactance of the series combination is thus given by:

If as stated, for some definite frequency the above reactance is equal to zero, then:

Now, for double the above frequency, the first term in the above reactance equation, is twice as large, While the second term is numerically only one-half in value. Assume that for the original frequency the positive reactance (22L) is 1000ohms, the negative reactance is now, for double frequency 500 ohms. Therefore, the net reactance is (2000- 500) ohms or only 1500 ohms. The neutralization which was for the frequency as originally assumed is now only 25% for double frequency. This brief description gives a clear idea of the distortion that is produced when capacity reactance is used in series with an inductance.

It also gives a clear picture of the great value of negative inductance, as herein disclosed, since neutralization is in the same proportion for all frequencies when negative inductance is used to balance out in part a positive inductance in an electrical network.

The discussion given above for series neutralization of the positive reactance of an inductance by means of negative inductance as compared with similar neutralization by means of capacity also holds true for parallel neutralization, or parallel use of negative inductance with positive inductance.

The method I have discovered to realize the object of this invention is briefly, to use in parallel with a negative resistance a series combination comprising an ordinary positive resistance having the same absolute value as the negative resistance and a capacity. The value of the capacity used is to be chosen in e accordance with the rules indicated beloW.-

The novel features of my invention are pointed out in the appended claims. The invention itself will be best understood by reference to the drawing and the discussion given hereunder.

Fig. 1 of the drawingshowsthe generalidea underlying the use of my method. Fig. 2 shows a particular method of carrying out my invention, using as a negative resistance the particular type of circuit and apparatus covered in U. S. A. Patent No. 1,606,350, granted to me. Fig. 3 shows in graphic form the characteristics of the negative inductance device as a function of frequency. As a matter of interest, the characteristics of an ordinary capacity are also shown in this same figure in dotted line. It will be seen at a glance how different the two are. In the case of the negative inductance of the present specifications, the absolute value of the negative inductance increases as the frequency increases. while for an ordinary capacity the opposite trend holds. Furthermore the reactance variation is directly proportional to frequency, while in the case of ordinary capacity it is inversely proportional to the frequency.

The arrangement of apparatus shown on Fig. 1 of the drawing across terminals (1, 2) represents a negative inductance within the meaning already specified. Element 3 is an ordinary resistance. Element 4 is a negative resistance. It may be of any type known to the art. It may be of the type first disclosed by Dr. Hull (U. S. A. Patent, 1,313,188), 01' preferably, it may be of the type first disclosed by me (U. S. A. Patent 1,606,350) and reproduced in Fig. 2 of the drawing Wired across terminals 7 and 8.

To-show how effectively the combination realizes the requirements for a negative inductance, as defined above, consider the mathematical expression for the impedance of the parallel combination of elements 4, 5 and 6.

Using the usual imaginary notation, the ex pression 1s given by multiplying the numerator and the denominator of the above expression for the parallel impedance of elements 4, 5 and 6, by (+j) (6) Parallel impedance The first term of the above expression is a negative resistance. The second or imaginary term represents a negative reactancc,

and is of opposite sign to the positive reactance of an inductance. Furthermore it can be written in the form:

It is seen, from the expression as just written, that the reactance of this new device is directly proportional to (p), the frequency factor, just as in the case of an ordinary inductance. But, it has the opposite sign instead of the positive sign (-1- which is the sign assumed in these specifications for the reactance of an ordinary inductance. Now (R) and (C) in the above expression are practically anything we choose. We can therefore, if we wish to secure a negative inductance of predetermined value (L) so choose the above values of (R) and (C) that We can secure therefore the value of the element (6) of the drawing from fundamen' tal Equation (7). It should be noted that R is still indeterminate and may be best chosen so as to secure convenient or practical values for the capacity (C) required to realize a negative reactance.

Once we have determined the value of (C), as just described, we can write as follows the value of the parallel impedance of elements (4) (5) and (6).

The combination of element 3 (series positive resistance It) with the parallel impedance of elements 4:, 5 and 6 as just written has an impedance equal to Parallel impedance R jpL.

' pletely the reactance sheet of this inductance,

regardless of the frequency of the current which may be transmitted through such a circuit if the constants of the circuit have been chosen as indicated to obtain a value of (-L) for the negative inductance.

A very valuable advantage of the arrangement shown on the drawing is the ease with which the value of the negative inductance may be changed. To obtain a change it is suflicient to change the value of the resistance elements shown on Fig. 1 or Fig. 2, without changing the condenser or capacity element (6). All the resistance elements must be changed the same amount. If the various resistance elements have been changed, say 10%, the inductance will have changed with the square of said change, or 1.21. This will be clear by reference to formula (7). Or we may leave the resistance elements alone, and change only the value of condenser (6). In this case, the variation of the Value of the negative inductance is directly proportional to the variation in the value of the capacity of element I claim:

1. An automatic device having reactance characteristics of opposite sign to that of an inductance, such reactance varying in direct proportion with frequency, in accordance with the formula:

Reactance 2w (L) where (f) is the frequency and (L) is a constant.

'2. A two-terminal network consisting of the combination of a negative resistance in parallel with a positive resistance and a condenser in series, said parallel combination being wired in series with a positive resistance, all three resistances being substantially equal in absolute value.

3. An automatic device having reactance characteristics of opposite sign to that of an inductance, such reactance varying in direct proportion with frequency, with additional means for varying the value of said reactance in equal proportion for all frequencies.

4. In an electric network containing an inductance, automatic means for the partial neutralization of said inductance to the flow of currents through said network, equally effective for all frequencies.

5. In an electric network for the simultaneous transmission of a band of frequencies, containing an inductance, automatic means for the complete neutralization of said inductance to the flow of currents through said network, equally eifective for all frequencies.

6. In an electric circuit containing an inductance, automatic means for neutralizing the effect of said inductance to the flow of current through said circuit, such means being equally effective at all frequencies.

In testimony whereof I affix my signature.

MIHRAN M. DOLMAGE. 

